1) Field of the invention
The present invention relates to a process for producing an xcex5-caprolactone which has excellent thermal coloring resistance and polycaprolactone which has low color.
2) Prior Art
xcex5-caprolactone is produced by oxidation of cyclohexanone. An oxidation method which uses an organic peroxyacid as oxidant and a co-oxidation method in which cyclohexanone is oxidized with an aldehyde are known. Peracetic acid or perpropionic acid is used as the organic peroxyacid in the oxidation method and acetaldehyde or benzaldehyde is used as the aldehyde in the co-oxidation method.
xcex5-caprolactone is polymerized and used for foaming materials, polyesterpolyols and biodegradable plastics etc. Improvement of xcex5-caprolactone quality is desired because high quality influences rate of polymerization and color of the product. In particular, purity, acid-value, water-content and thermal coloring resistance influence the color of the polymer and rate of polymerization Japanese Laid-open patent 11-158172 (1999) describes xcex5-caprolactone with less of a low boiling point component and high purity and having lower coloring at the stage of producing or storing of monomer and producing or using of polymers.
However, xcex5-caprolactone which has excellent thermal coloring resistance and polycaprolactone which has low color were not produced using xcex5-caprolactone having fewer low boiling point components and higher purity during co-xidation of cyclohexanone and aldehyde by our study because the low boiling point components in the co-oxidation method are different from those in the oxidation using peroxyacid.
It is an object of the present invention to provide a process for producing xcex5-caprolactone which has excellent thermal coloring resistance from co-oxidation of cyclohexanone and aldehyde, and to provide polycaprolactone which has low coloring.
The present inventors assiduously conducted investigations to solve the above problems, and found that (1) a small amount of high boiling point component which influences thermal coloring resistance and is difficult to separate is produced during co-oxidation of cyclohexanone and aldehyde, (2) the high boiling point component which influences thermal coloring resistance becomes possible to separate by introducing oxygen containing gas into the xcex5-caprolactone in the presence of cobalt, (3) xcex5-caprolactone which has excellent thermal coloring resistance is produced by introducing oxygen containing gas therein and removing low boiling point carboxylic acids, to obtain xcex5-caprolactone having acid values of lower than 0.15 mgKOH/g, (4) low color polycaprolactone is produced using this xcex5-caprolactone.
Thus, the present invention provides a process for producing xcex5-caprolactone by co-oxidation of cyclohexanone and an aldehyde to form a reaction mixture containing xcex5-caprolactone, low boiling components, high boiling components and color components, wherein the reaction mixture is subjected to an oxidation treatment with an oxygen containing gas in the presence of cobalt to convert color components to high boiling components and then separating the xcex5-caprolactone from the low and high boiling components, whereby the xcex5-caprolactone recovered has an acid value of less than 0.15 mg KOH/g.
The aliphatic aldehyde used in the co-oxidation method may be acetaldehyde, propionaldehyde or butyraldehyde. The aromatic aldehyde generally used in the co-oxidation method may be benzaldehyde, tolualdehyde, dimethylbenzaldehyde, trimethylbenzaldehyde, ethylbenzaldehyde, cuminaldehyde, butylbenzaldehyde, methoxybenzaldehyde, phenoxybenzaldehyde, cyclohexylbenzaldehyde and biphenylaldehyde. When these aliphatic aldehydes or aromatic aldehydes possess isomeric forms, each isomer or the mixture thereof may be used.
Purified xcex5-caprolactone is produced by removing impurities from the reaction products of co-oxidation. When acetaldehyde is used as the aliphatic aldehyde, the reaction mixture contains xcex5-caprolactone, unreacted cyclohexanone, acetic acid, acetaldehyde, adipic acid, caprolactone oligomer, caprolactone polymer and oxycaproic acid etc. When 2,4-dimethylbenzaldehyde is used as the aromatic aldehyde, the reaction mixture contains xcex5-caprolactone, unreacted cyclohexanone, 2,4-dimethylbenzoic acid, 2,4-dimethylbenzaldehyde, adipic acid, caprolactone oligomer, caprolactone polymer and oxycaproic acid etc.
When acetaldehyde is used as the aliphatic aldehyde in distillation purification of the reaction mixture, the high boiling point component of by-product such as adipic acid) caprolactone oligomer, caprolactone polymer and oxycaproic acid is removed at first, then the low boiling point component such as unreacted cyclohexanone, acetic acid and acetaldehyde is removed. When 2,4-dimethylbenzaldehyde is used as the aldehyde, unreacted cyclohexanone (boiling point 155.6xc2x0 C.) is separated at first, then the high boiling point component of 2,4-dimethylbenzoic acid (boiling point 267xc2x0 C.), 2,4-dimethylbenzaldehyde (boiling point 225xc2x0 C.) is removed. used as the aldehyde, unreacted cyclohexanone (boiling point 155.6xc2x0 C.) is separated at first, then the high boiling point component of 2,4-dimethylbenzoic acid (boiling point 267xc2x0 C.), 2,4-dimethylbenzaldehyde (boiling point 225xc2x0 C.) is removed.
In the present invention, the reaction mixture from the oxidation product of cyclohexanone is purified by the above method. Then, the remaining high boiling point component is removed by distillation The xcex5-caprolactone thus obtained is purified by introducing oxygen containing gas in the presence of cobalt (which is referred as xe2x80x9coxidation treatmentxe2x80x9d), then changing the color component to a high boiling point component and removing a small amount of low boiling point acids by distillation to obtain the xcex5-caprolactone product (boiling point 235.3xc2x0 C.).
These methods of distillation purification are performed by known methods at as low a temperature and as low a pressure as possible to avoid change in quality during distillation.
In the present invention, the temperature of the oxygen containing gas introduced to the xcex5-caprolactone separated by distillation, is in the range of 80-200xc2x0 C., and preferably 100-180xc2x0 C. . When the temperature is too low, a long time is required to change the color component, which is difficult to separate from xcex5-caprolactone, to a high boiling point component. When the temperature is too high, xcex5-caprolactone may be polymerized and may produce low boiling point aliphtic acids and increase the acid value.
In the present invention, the color component is changed to a high boiling point component by contacting xcex5-caprolactone with oxygen containing gas in the presence of cobalt. Decomposition of xcex5-caprolactone and generation of low boiling point carboxylic acid are restrained to decrease the acid value in the presence of cobalt.
Cobalt compounds which are soluble in xcex5-caprolactone such as cobalt naphtate or cobalt octylate are used in the oxidation treatment. The amount of cobalt added to xcex5-caprolactone is 0.001-10 ppm by weight, and preferably 0.01-5 ppm by weight. When the cobalt content is too low, decomposition of xcex5-caprolactone and generation of low boiling point carboxylic acids are increased which decreases the acid value, and colored polycaprolactone is produced when xcex5-caprolactone thus oxidation treated is polymerized. When cobalt content is too high, xcex5-caprolactone is apt to polymerize spontaneously.
Generally, air is used for the oxygen containing gas and the pressure of the reaction is from one atmosphere to 10 kg/cm2. Oxygen containing gas is introduced continuously and xcex5-caprolactone accompanied by vent gas is recovered by condensation. The pressure of oxygen introduced to the xcex5-caprolactone is 0.0002-1.0 kg/cm2, and preferably 0.0005-0.1 kg/cm2. Oxygen pressure in the contacting tank is calculated from the concentration of oxygen in vent gas from the condenser. When the oxygen pressure is too high, decomposition of xcex5-caprolactone and generation of low boiling point carboxylic acids are increased, increasing acid value. When the oxygen pressure is too low, the problem of thermal coloring resistance may tend to occur because the conversion of coloring components to high boiling point component is decreased.
The mol-ratio of oxygen to xcex5-caprolactone is in the range of 0.0001-0.030. The amount of oxygen used is controlled to keep the acid value within 0.3 mgKOH/g by oxidation treatment when xcex5-caprolactone is decomposed to low boiling point carboxylic acids. The time of the oxidation treatment is from 5 minutes to 10 hours and preferably from 15 minutes to 7 hours.
By contact of xcex5-caprolactone with oxygen containing gas in the presence of cobalt according to the above method, the coloring component which is present in a significant amount in xcex5-caprolactone is changed to a high boiling point component. Formation of by product low boiling point carboxylic acids that cause coloring of the polymer is decreased. Components that cause coloring become high boiling point components which are separated easily by simple distillation or general distillation because the difference of boiling point between xcex5-caprolactone and the component is large. xcex5-caprolactone which is separated from the purification column is contacted with oxygen containing gas because large amounts of coloring component are produced by this treatment when xcex5-caprolactone contains a large amount of impurity.
Though low boiling point carboxylic acid is produced when the coloring component is oxidized to form a high boiling point component, the acid value of xcex5-caprolactone for oxidation treatment should be lower than 5 mgKOH/g. This xcex5-caprolactone is distilled again if necessary. As low boiling point carboxylic acids such as acetic acid, propionic acid, butyric acid and pentanoic acid possess large differences in boiling point; these low boiling point carboxylic acid are separated by simple distillation or purification distillation with reflux if necessary.
After xcex5-caprolactone is separated from the distillation column with introduction of oxygen containing gas, coloring components are removed by re-distillation and excellent thermal color resistant xcex5-caprolactone is obtained. Low color polycaprolactone is produced from this xcex5-caprolactone.
Though these treatments may be carried out batch-wise, semi-continuously or continuously, continuous method is preferable.
Polycaprolactone is used in many fields industrially according to the average molecular weight and functional groups contained. For example, polycaprolactone of a molecular weight of 500-5000 with glycol as starter is used as a raw material for polyurethane and paint. Polycaprolactone containing a double bond suitable for radical polymerization is used for coating materials for car and home electric products. Polycaprolactones possessing molecular weights of over 10000 are used for plastic forming products, film and hot melting adhesives. These polycaprolactones are mainly produced by polymerization using compounds containing hydroxy-functional groups as polymerization starters.
Compounds containing active hydrogen except water, such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,3-butylenediol, diethylene glycol, neopentyl glycol, 1,6-hexanediol, trimethyrolpropane, and pentaerythritol are used as polymerization starting materials to produce polycaprolactone.
The kind and amount of the starter is decided by use of polycaprolactone resin.
For the catalyst to produce polycaprolactone, general catalysts for ring opening, adduct or polymerization reactions may be used. Concretely inorganic bases, inorganic acids, organic bases metal catalysts, tin compounds, titanium compounds, aluminum compounds, zinc compounds, molybdenum compounds and zirconium compounds are used. Among them, from the standpoint of handling, harmlessness, reactivity, colorlessness and stability; tin compounds or titanium compounds are preferable. As the tin compound of the present invention, a monobutyl tin compound such as tin(l)octyl acid, monobutyl tin oxide, monobutyl tin tri(2-ethylhexanate); dibutyl tin compounds such as dibutyl tin oxide, diisobutyl tin oxide, dibutyl tin acetate, di-n-butyl tin di-laurate; and titanium compound such as tetramethyl-titanate, tetraethyl-titanate, tetra-n-propyltitanate, tetraisopropyl-titanate, and tetrabutyl-titanate are used. These compounds may be used independently or as mixtures.
The temperature of polymerization to produce polycaprolactone is 50-250xc2x0 C., preferably 90-220xc2x0 C., and more preferably 100-200xc2x0 C . When the temperature is lower than 50xc2x0 C., the rate of polymerization to polycaprolactone is too low. When the temperature is higher than 250xc2x0 C., polycaprolactone may decompose causing coloring and producing byproducts.
For the polymerization reactor of polycaprolactone, known reactors such as batch reactors with impellers, continuous or semi-continuous reactors, kneader type mixers, screw type mixers, static mixer type reactors and reactors connected to them continuously may be used.
The water content of xcex5-caprolactone for polymerization is 0.5% or less by weight, preferably 0.1% or less by weight, and more preferably 0.03% or less by weight. Dilute solution or stabilizers may be used in the polymerization of xcex5-caprolactone in the present invention.
According to this invention, as disclosed in the following Examples, by removing high boiling point components by distillation from xcex5-caprolactone separated from the purifying column after introduction of oxygen containing gas in the presence of cobalt, xcex5-caprolactone which has excellent thermal coloring resistance is obtained. From xcex5-caprolactone obtained by the present invention, polycaprolactone which has low coloring may be produced and high quality of polycaprolactone product may be obtained.